Echinoderms:

Sea Stars, Crinoids, Brittle Stars, Basket Stars,

Sea Cucumbers, Sea Urchins and Sand Dollars

Story and photos by Marty Snyderman

Phylum:
Echinodermata (spiny-skin)

Note: This article is the seventh in a series
that elaborates on the natural history of the marine
animals that you and your pals are likely to
encounter when diving. The pieces are presented in
order according to the taxonomic system used by
scientists when they describe and classify plants
and animals. An overview of the articles and the
taxonomic system is provided in the February 2001
issue.

Whether you are a newly certified diver (if so,
we offer our congratulations) or an old salt (good
for you, too), odds are you are familiar with the
animals known as echinoderms. Of course, you might
not realize it because most of us use other names
when referring to the animals described in the
phylum Echinodermata (ik I NO DERM at uh). We use
names such as sea star, sea cucumber, sea urchin and
sand dollar. So your familiarity with the members of
this phylum might range from the joy of first
learning about sand dollars to the pain of getting
punctured by a sea urchin. In between you probably
have marveled at the beauty of a brightly colored
crinoid or feeding basket star and made a
questioning face upon seeing a sea cucumber.

If you are like a lot of people, you recognize
many of these animals but don’t realize that they
are closely related to one another and to brittle
stars, crinoids (also known as feather stars and sea
lilies) and basket stars. Well, live and keep on
learning. A good diver always does.

An ancient group of marine animals, echinoderms
are very well-known from fossil records. Today,
there are about 6,000 living species and all are
bottom dwelling, or benthic, animals that live in
saltwater ecosystems. Most echinoderms occur in
temperate seas, but the phylum is represented in
tropical and polar seas, too.

The Big Picture

The
word “echinoderm”(ik I NO DERM) is derived from the
Greek “echinos,” meaning spiny, and “derma,” meaning
skin. The skin of most species is covered by spines,
“warts,” or other projections. When first seeing an
echinoderm, most people think that the skin is tough
because in many species the skin looks tough.
However, like books, you don’t want to judge an
echinoderm by its cover, as the skin in many species
is surprisingly delicate.

To many people, echinoderms look like relatively
simple animals, and it comes as a bit of a surprise
to discover just how sophisticated they are. The
skin of echinoderms is filled with thousands of
nerves. The skin and the nervous system help protect
the internal skeleton, which in many species
consists of calcium-impregnated plates. As the
animal grows, the plates expand to support and shape
the body. The spines found in many species are
attached to those plates. The bodies of some species
are covered by tiny, pincer-like organs called
pedicellariae (ped-uh-sell-air-ee-uh) which are used
in defense and to help echinoderms clean their own
skin. Anyone who has ever committed the “no-no” of
picking up a sea star is probably familiar with
these organs. The small pincers are quick to grab
on, and their numbers and tenacity make it difficult
to let go of the animal without hurting it. One more
reason not to handle or harass marine life.

Other prominent characteristics found in
echinoderms are five-part radial symmetry, distinct
organs, including tube feet and the absence of a
true brain. Obvious five-part radial symmetry is
easily observed in most sea stars but you need to
examine the internal body structure of creatures
like sea cucumbers and crinoids to see the symmetry,
or be willing to accept some statements on good
faith.

Even though the nervous system of echinoderms is
built from several simple nerve rings that lack a
“central command headquarters,” members of the
phylum are capable of some surprisingly complex
response patterns. For example, echinoderms crawl
around on the substrate on thousands of small tube
feet which are part of a body known as a
water-vascular system. Allowing purposeful movement,
the system is well-coordinated, but slow.

When echinoderms move in a manner such as
walking or crawling, they pump sea water through a
series of internal body canals. The water is used to
inflate some of the tube feet, causing them to
expand. In many species, the tube feet are equipped
with suckers that grip onto the sea floor. The feet
hold tight to the bottom as muscles within the feet
contract, enabling the animal to propel itself.
Repeating this process allows echinoderms to travel
in a fashion that is “more turtle than hare.”

One of the most remarkable traits about
echinoderms is their ability to regenerate body
parts that have been lost. Many tropical
destinations have learned this lesson the hard way.
A sea star known as the crown of thorns readily
preys upon living corals, and a crown of thorns
invasion poses a serious, but usually temporary
threat, to coral reef ecosystems. A few decades ago,
when crown of thorns showed up in big numbers,
resort owners asked local divers to go into the
water and chop them up. Bad idea. Like many sea
stars and brittle stars, crown of thorns not only
regenerated lost parts, but in some instances the
lost part often regenerated an entirely new animal.
The not-surprising lesson here, “Don’t mess around
with Mother Nature.” The ability to regenerate is an
important part of the survival strategy of many
species as they readily sacrifice body parts to
escape predation.

Sexes are separate in most echinoderms, but they
look very similar and distinguishing male from
female is a task best left to specialists.
Reproduction is most often achieved by spawning as
males and females simultaneously release sperm and
eggs into the water column where sexual union
occurs. The resulting larval life-forms spend from a
few days to a few weeks as part of the community of
plankton before settling down to live their adult
lives on the sea floor.

Amazingly, some echinoderms can reproduce
asexually on a regular basis. These “sexperts” break
in two and regenerate into two animals.

Five Classes

Within their phylum, modern day echinoderms are
described in five classes. They are listed below.

Crinoidea
(crinoids and sea lilies). [cry NOID E uh]
Different species of crinoids possess anywhere from
five to 200 arms. When first developing, crinoids
have only a few arms, but some species develop more
as they grow. The arms of many species are branched
into small structures known as pinnules. Crinoids
are suspension feeders; while they lack a truly
organized “filter,” they collect plankton and dead
organic matter from the water column.

Asteroidea
(sea stars). [Aster OID E uh] Though there are
some notable exceptions, the pentamerous symmetry of
sea stars is usually obvious. They possess five
arms. The arms of sea stars are not as sharply set
off from the body center as are those of brittle
stars. Some sea stars are scavenger and predator,
while others are one or the other.

Ophiuroidea
(brittle stars). [O FE yur OID E uh] Brittle
stars possess long, thin, fragile arms set off
sharply from the center of the body, which is known
as a central disc. In most species the arms fall off
easily when the animal is under attack. Members of
this class include a variety of suspension feeders
and species known as detritovores that feed on dead
and decaying matter, and predators.

Holothuridae
(sea cucumbers). [hollow THUR id ay] Sea
cucumbers possess cylindrically shaped, pliable
bodies. The characteristic of radial symmetry is
only obvious in a cross-sectional view. Some sea
cucumbers are sediment ingestors, some feed on
detritus and others are suspension-feeders. When
threatened, many sea cucumbers are able to expel
their innards, an escape strategy which repels many
would-be predators.

Echinoidea
(sea urchins and sand dollars). [ech in OID E
uh] Most members of this class (known as echinoids)
have bodies that are flat and disc-shaped or round
with spines. Most species are herbivores and
sediment ingestors.

Now, let’s take a closer look at each class...

Crinoids

Crinoids are considered among the most ancient
of all marine creatures. When they appeared in the
world’s oceans eons ago, crinoids were attached to
the sea floor via large stalks. Ancient crinoids are
also known as “sea lilies.” To feed, these organisms
reached up into the plankton-rich water column to
capture their prey. Many believe that crinoids
needed more mobility to compete for and catch their
prey, and to escape predation. As a result, most
lost their stalks and eventually developed numerous,
shorter claw-like “legs” known as cirri (also
cirripedia) which offer greatly increased mobility
and enable the animals to grip the sea floor.
Modern-day crinoids, also called feather stars, tend
to “walk” along the bottom, but at times you will
see them “swimming” through the water in a slow,
graceful, undulating motion.

A few species of stalked crinoids have survived,
and scientists jokingly call them “living fossils.”
Stalked crinoids are rare and live in water that is
deeper than sport divers explore. However, you will
likely see a stalked crinoid or two if you go on one
of the deeper submarine rides offered in Grand
Cayman.

Modern-day crinoids occur in oceans around the
world, and they do so with eye-catching style and
color. Crinoids occur in brilliant colors from
bright-daisy yellow to gold, green, bright red,
white, black and all kinds of color combinations.
All are feather-like as the arms of these delicate
creatures project out from their perch like the
plume of a peacock. When feeding, which is at night
in most instances, many species look like a basket
made of long, colorful, feathered branches.

In daylight, divers often see only the ends of
the delicate, feathery arms reaching out of a
crevice. But at night, especially in tropical seas,
crinoids can be seen perched high atop sea fans and
coral heads where their arms can easily reach into
food-bearing currents. Crinoids are known as
suspension-feeders. They reach into the water column
and, using their tube feet, capture their prey of
small planktonic organisms.

The arms of feather stars are branched into
numerous small structures called pinnules, and each
pinnule contains numerous small feet. Pinnules are
also covered with sticky secretions which trap food
that is then grabbed by the tube feet. At that point
the arm is curled inward toward the animal’s mouth.

Many crinoids are stunning. It is quite tempting
to reach out and touch them. Please don’t. They are
far more delicate than they appear, and their arms
are quick to fall off even if only gently handled.

Sea Stars

Depending upon your age, you probably know the
group of animals correctly called sea stars either
as starfish or sea stars. I have to confess that
these animals were starfishes when I was a kid.
Obviously, they are not fish, so a name change was
generally agreed upon in educational circles to try
to prevent confusion. But no matter what name you
use, colorful sea stars add a lot of interest to
many dives. Worldwide, there are about 1,500 species
of sea stars. Most occur in tropical and temperate
seas where they serve important roles as marine
predators. Of the sea stars found in the tropics,
most occur in the Indo-Pacific, but several species
play important environmental roles in the waters
surrounding Florida, the Bahamas and the island
nations of the Caribbean.

The arms (sometimes called rays) of sea stars
are much thicker and more prominent than those of
brittle stars. Most species possess five arms, but
members of some sea star families have many more.
The giant sunflower star, which occurs along the
Pacific coast of the western Americas, is a
well-known example of a many-armed sea star.

The mouth of sea stars is on the underside of
the central disc. Neatly arranged rows of tube feet
can be found on the underside of each arm. The mouth
of some echinoderms such as crinoids is on the upper
surface of the body, but sea stars find their food
on the sea floor and having a mouth on the bottom
surface has its advantages for bottom feeders.

Sea stars prey on a variety of food sources.
Some sea stars have developed the ability to prey
upon a variety of bivalves (For more information,
see “Mollusks,” Dive Training, June 2001.) These sea
stars grasp the shells of the bivalves with the
suckers on their tube feet and then … what happens
next is still debated. Some specialists believe the
sea stars pull the shells apart and then extrude
their own stomachs through the shells of the bivalve
and begin to digest their prey externally. Others
maintain that the sea stars first tire the mollusk
by holding the shells closed, and eventually this
forces the bivalve to open its shells to access
oxygenated water. Cooperative prey make it much
easier for the sea star to get to the desired body
parts. Those who argue against this theory are quick
to point out that the shells of bivalves rarely
close perfectly anyhow, so getting a fresh supply of
oxygenated water is probably not impossible for
bivalves. Most sea stars do not feed in this manner,
but those that do can push their stomachs through an
opening that is only .04 inches (10 cm) wide.

The species of sea star known as the crown of
thorns is, perhaps, the most widely recognized sea
star. Armed with long, thorn-like spikes, this
species is well-known because of its voracious
feeding habits and the manner in which hordes of
crown of thorns are known to devastate corals reefs
by preying upon the living corals.

Brittle Stars and Basket Stars

Even after learning that brittle stars and
basket stars are echinoderms, you might not suspect
that they are so closely related that they would be
described in the same class. But their body design
is quite similar and they are very closely related
animals. Their bodies are made of a central disc
that is distinctly separated from slender arms. But
basket stars have many more arms than brittle stars,
which usually have five.

Brittle stars and basket stars tend to be
reclusive during the day, and unless you are an
experienced diver and a skilled observer, you might
think that the reef you are exploring during a day
dive is not a place these animals call home. But
visit the same reef at night, and you will think you
must be at a different site as often there are
brittle stars and basket stars everywhere you look.

In many temperate and tropical locales, brittle
stars seem to occupy almost every conceivable nook
and cranny. Many bury themselves in the sand and all
you see during the day is what looks like a waving
arm of some creature that must have been
accidentally covered up. Others hide in crevices in
the reef. In tropical seas, if you look in and
around the base of sponges, you will likely find a
bunch of scrunched up, hiding brittle stars. They
shy away from light and are mostly seen at night.
And even at night, if you do want to take a close
look, it is often a good idea only to illuminate
them with the peripheral beam of your dive light. If
alarmed, most brittle stars are surprisingly agile
and fast to seek cover.

Every once in a while, brittle star populations
seem to explode in some areas and can be seen all
over the place day and night. In fact, a few years
ago, during the last El Niño event in southern
California, we experienced an enormous brittle star
explosion. Almost everywhere anyone dived, there
were uncountable numbers of very colorful brittle
stars, including on top of sea fans, urchins, sea
stars and on the reef and sand.

The arms of brittle stars are lined with
numerous spines that are arranged in rows, and they
have several tube feet distributed along the rows.
In some species, the dense concentration of spines
gives the brittle stars a fuzzy appearance.

Brittle stars feed in a variety of ways. Some
are suspension feeders, capturing plankton out of
the water. Others sift through sand seeking organic
debris, while others are predators that pursue a
variety of small invertebrates and vertebrates.

Basket stars only have five arms. However, the
arms are branched at the base so with many specimens
it seems like they have dozens of arms. Of course,
part of the reason is that the branches are
branched, too. The repeated branching eventually
makes a creature that looks like a large, somewhat
tangled ball of yarn. That is the nonfeeding daytime
look. When basket stars feed the ball of yarn
unfurls and the arms extend to make a basket-like or
net-like sieve that catches plankton and other
organic matter that passes. Tube feet arranged in
regular patterns along each of the arms help basket
stars catch their food and pass the food particles
to their mouth. When feeding, basket stars are
usually perched at prominent outcroppings or in high
places in a reef community, and often they are seen
on sea fans.

Sea Cucumbers

Aptly named, sea cucumbers are the sluggish
creatures that often cause new divers to ask, “What
are those things on the sea floor that look like
cucumbers?” They differ from other echinoderms in
that they are more soft-bodied and their leathery
skin lacks spines, but the skin of many species is
covered with wart-like projections. Their five-sided
radial symmetry is only obvious if you view a
cross-section of an animal in a lab and see five,
elongated, radial muscles. Interesting to know
perhaps, but not the sort of thing most of us really
want to do with our time.

In a word or two, sea cucumbers are the ocean’s
vacuum cleaners. They use brush-like mouths to
ingest organic matter from sediment on the sea
floor. At one end of the sea cucumber’s
sausage-shaped body is its mouth; at the other end,
its anus. The mouth lacks teeth, but contains from
eight to 30 mop-like tentacles that are used to trap
prey and draw it into the mouth. In some species,
the tentacles also secrete a mucus net that is used
to trap prey. They are the long, whitish, stringy
looking fibers that extend from crevices and look
like they are connected to nothing. When feeding,
each tentacle is periodically wiped off in the
esophagus to remove food and to replace the mucus
coating. As they feed and move along the sea floor,
sea cucumbers often leave neat rows of excreted
inorganic deposits behind them.

A relatively small (thank goodness) species of
fish known as a pearl fish actually lives in the
bodies of sea cucumbers, exiting and entering at
will through the cucumber’s anus.

When threatened, some sea cucumbers can
eviscerate or expel their own stomachs in an effort
to repel or distract predators. That’s a small price
to pay to escape predation, especially when they can
regenerate a new stomach within a matter of days.
When relaxed, the bodies of sea cucumbers are
flexible and elongate, but when threatened or
disturbed they are able to shorten and harden their
bodies.

Some species have tube feet scattered around a
rather uniform looking body, while in other species
the feet are distributed only in five lateral
grooves. Some species have one body surface that is
slightly flattened, and all of the tube feet are on
that flattened side.

Sea cucumbers are highly valued as a food source
in many parts of the Far East, and commercial
fishing industries pose serious threats to sea
cucumber populations in many parts of the world as
the catch from these fisheries is primarily shipped
to markets in the Orient.

Sea Urchins and Sand Dollars

This class of echinoderms is somewhat of a
dichotomy. Most divers probably get warm, fuzzy
feelings when we think about sand dollars and the
places where we first saw them as kids. But say the
term “sea urchin” to scuba divers and most think of
them as #@!!%! painful pin cushions. Few marine
organisms have inflicted as much pain, grief and
misery on divers, snorkelers and even beachgoers as
sea urchins have with their needle-like spines. If
you closely examine the spines, perhaps even under a
microscope, you will see that the spines are armed
with several fishhook-like barbs that help the
spines penetrate easily but make them difficult to
remove. So while we tend to think of these creatures
in very different lights, scientists know them as
very closely related animals.

The bodies of sea urchins and sand dollars
display five-part symmetry, and members of each
group possess five rows of rather neatly arranged
tube feet that protrude through hard, calcified
plates. The fused plates can easily be seen when you
look at the “skeleton” or test, as it is more
properly known. The spines of some urchins, such as
the long-spined urchin found in the Caribbean and
some close Indo-Pacific cousins, are needle-sharp
and can be 12-14 inches long, or longer. The spines
of purple sea urchins and black sea urchins found in
reefs of southern California are not as long or as
sharp, but they, too, are painful and should be
avoided.

Despite what divers injured by sea urchins might
think, the spines are primarily used to ward off
predators that would like to get to the meat inside
the urchin’s test. Cleverly, some small fishes
ranging from a variety of gobies to juvenile
seabasses use the forest of spines as a perfect
place to hide from potential predators. Some shrimps
also associate with sea urchins.

Fortunately, most species of long-spined sea
urchins tend to occupy crevices during the day, and
emerge at night to feed.

The spines of a variety of other species are
less than 1 inch long and in some animals appear to
be missing, or impotent. Many of these urchins cover
their bodies with organic debris and rocks for added
protection. Some of these species are poisonous to
handle.

Many urchins are voracious feeders and they prey
heavily on a variety of algae, including giant kelp.
In turn, urchins are heavily preyed upon by a
variety of fishes and sea otters. It is normal to
see a fish such as a California sheephead or a queen
triggerfish in Caribbean waters swimming around with
a sea urchin spine or two stuck into its head.
Again, there is no such thing as a free lunch. With
the removal of otters because of overhunting in
California, sea urchin populations occasionally
expand rapidly and that often means that the urchins
annihilate kelp forests.

In contrast to other echinoderms, sand dollars
have flat, disc-shaped bodies. But if you closely
compare their bodies with those of sea urchins,
several common characteristics soon become obvious.
Both groups walk across the sea floor using tube
feet, though some urchins gain additional thrust by
using their spines. It is quite common to find
hundreds of sand dollars within moments of
discovering a single specimen.

This article concludes the part of this series
that deals with invertebrates. The remaining
articles will delve into the natural history of many
marine vertebrates.